feat: RGB LED support — blinks green on RX, red on TX

- Added RgbLed trait with impl on () for boards without LED
- Added WS2812 driver (driver/ws2812.rs) using ESP32-S3 RMT peripheral
  - 80MHz clock, 12.5ns resolution, 24-bit GRB encoding
  - Zero extra crate dependencies
- Heltec V3/V3-UART/V4: WS2812B on GPIO38 via RMT channel 0
- RAK WisBlock/Wio Tracker: type LedDriver = (), led = None (no-op)
- Display task: green blink on RX packet, red blink on TX packet
- LED auto-off on timer tick (~1s), off when display is off/reset
- LoRaBoard trait gains type LedDriver: RgbLed associated type
- BoardParts gains led: Option<L> field

src/board/traits.rs

@@ -1,14 +1,23 @@
 //! Board abstraction trait.
 //!
 //! Every board must implement [`LoRaBoard`] with concrete associated types
-//! for its radio, host communication, and display peripherals. The compiler
-//! enforces completeness — missing types or methods are compile errors.
+//! for its radio, host communication, display, and LED peripherals. The
+//! compiler enforces completeness — missing types or methods are compile errors.
 //!
 //! See `src/board/PORTING.md` for a step-by-step guide to adding a new board.
 
 use embedded_graphics::draw_target::DrawTarget;
 use embedded_graphics::pixelcolor::BinaryColor;
 
+/// Async RGB LED control. Boards without an LED use `()` which is a no-op.
+pub trait RgbLed {
+    fn set_rgb(&mut self, r: u8, g: u8, b: u8) -> impl core::future::Future<Output = ()>;
+}
+
+impl RgbLed for () {
+    async fn set_rgb(&mut self, _r: u8, _g: u8, _b: u8) {}
+}
+
 /// Compile-time contract for a DongLoRa board.
 pub trait LoRaBoard: Sized {
     /// Human-readable board name (shown on display splash screen).
@@ -29,6 +38,9 @@ pub trait LoRaBoard: Sized {
     /// Concrete display driver type (must implement DrawTarget for rendering).
     type DisplayDriver: DrawTarget<Color = BinaryColor>;
 
+    /// RGB LED driver. Boards without an LED use `()`.
+    type LedDriver: RgbLed;
+
     /// Initialize the board hardware.
     fn init() -> Self;
 
@@ -36,13 +48,14 @@ pub trait LoRaBoard: Sized {
     fn mac_address() -> [u8; 6];
 
     /// Decompose initialized board into peripheral bundles for each task.
-    fn into_parts(self) -> BoardParts<Self::RadioParts, Self::CommParts, Self::DisplayParts>;
+    fn into_parts(self) -> BoardParts<Self::RadioParts, Self::CommParts, Self::DisplayParts, Self::LedDriver>;
 }
 
 /// Peripheral bundles for each firmware task, produced by [`LoRaBoard::into_parts`].
-pub struct BoardParts<R, C, D> {
+pub struct BoardParts<R, C, D, L> {
     pub radio: R,
     pub host: C,
     pub display: Option<D>,
+    pub led: Option<L>,
     pub mac: [u8; 6],
 }